X-ray Imaging System
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Radiologic Physics
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Three Principal Parts of an X- ray Imaging System
1. x-ray tube
2. operating console
3. high-voltage generator
OPERATING CONSOLE
The part of the x-ray imaging system most
familiar to the radiologic technologist is the operating console. The
operating console allows the radiologic technologist to control the
x-ray tube current and voltage so that the useful x-ray beam is of
proper quantity and quality.
Radiation quantity refers to the number of
x-rays or the intensity of the x-ray beam. Radiation quantity is usually
expressed in milliroentgens (mR) or milliroentgens/milliampere-second
(mR/mAs). Radiation quality refers to the penetrability of the x-ray
beam and is expressed in kilovolt peak (kVp) or, more precisely,
half-value layer (HVL).
The operating console usually provides for
control of line compensation, kVp, mA, and exposure time. Meters are
provided for monitoring kVp, mA, and exposure time. Some consoles also
provide a meter for mAs. Imaging systems that incorporate automatic
exposure control (AEC) may have separate controls for mAs.
Most operating consoles are based on computer
technology. Controls and meters are digital, and techniques are selected
with a touch screen. Numeric technique selection is sometimes replaced
by icons indicating body part, size, and shape. Many of the features are
automatic, but the radiologic technologist must know their purpose and
proper use.
Most x-ray imaging systems are designed to
operate on 220 V power, although some can operate on 110 V or 440 V.
Unfortunately, electric power companies are not capable of providing 220
V accurately and continuously.
Typical operating console to control an overhead radiographic imaging system. Numbers of meters and controls depend on the complexity of the console. (Courtesy General Electric Medical Systems.)
The line compensator measures the voltage
provided to the x-ray imaging system and adjusts that voltage to
precisely 220 V. Older units required technologists to adjust the supply
voltage while observing a line voltage meter. Today's x-ray imaging
systems have automatic line compensation and hence have no meter.
AUTOTRANSFORMER
The power supplied to the x-ray imaging system is delivered first to the autotransformer. The voltage supplied from the autotransformer to the high-voltage transformer is controlled but variable. It is much safer and easier to control a low voltage and then increase it than to increase a low voltage to the kilovolt level and then control its magnitude.The autotransformer works on the principle of electromagnetic induction but is very different from the conventional transformer. It has only one winding and one core. This single winding has a number of connections along its length.
AUTOTRANSFORMER LAW
- where
- Vp =the primary voltage
- Vs =the secondary voltage
- Np =the number of windings enclosed by primary connections
- Ns =the number of windings enclosed by secondary connections
Adjustment of Kilovolt Peak (kVp)
Some older x-ray operating consoles have adjustment controls labeled major kVp and minor kVp; by selecting a combination of these controls, the radiologic technologist can provide precisely the required kilovolt peak. The minor kilovolt peak adjustment “fine tunes” the selected technique. The major kilovolt peak adjustment and the minor kilovolt peak adjustment represent two separate series of connections on the autotransformer.Control of Milliamperage (mA)
The x-ray tube current, crossing from cathode to anode, is measured in milliamperes (mA). The number of electrons emitted by the filament is determined by the temperature of the filament.Filament circuit for dual-filament x-ray tube.
The filament temperature is in turn controlled
by the filament current, which is measured in amperes (A). As filament
current increases, the filament becomes hotter and more electrons are
released by thermionic emission. Filaments normally operate at currents
of 3 to 6 A.
A correction circuit has to be incorporated to counteract the space charge effect.
As the kVp is raised, the anode becomes more attractive to those
electrons that would not have enough energy to leave the filament area.
These electrons also join the electron stream, which effectively
increases the mA with kVp.
Thermionic emission is the release of electrons from a heated filament.
The product of x-ray tube current (mA) and exposure (s) is mAs, which is also electrostatic charge (C).
Filament Transformer
The full title for this transformer is the
Filament Heating Isolation Step-down Transformer. It steps down the
voltage to approximately 12 V and provides the current to heat the
filament. Because the secondary windings are connected to the high
voltage supply for the x-ray tube, the secondary windings are heavily
insulated from the primary.
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